This invention relates to a new and improved method for the purification of II-VI crystals. The preparation of such II-VI compounds as ZnTe, ZnSe, ZnS, and CdS by such well known techniques as melt growth or vapor-phase growth techniques are well known in the art.
In either case the materials are in contact with refractory container materials at high temperatures for considerable lengths of time and thus are subject to contamination by diffusion of impurities from the container material into the crystal. As a result these materials are frequently unsuitable for controlled doping.
An attempt to overcome this problem was reported by M. Aven et al, Applied Physics Letters, Vol. 1, No. 3, Nov. 1, 1962, pages 53-54. In this method the II-VI crystal, such as ZnTe or ZnSe, was purified by sealing the crystal, together with a solvent for the impurity, in a small quartz tube under vacuum and heating it for a time sufficient to establish equilibrium between the solid and the liquid phases. Thus, for ZnTe a molten zinc solvent was employed. Temperatures of between 900.degree. and 1000.degree. C. and firing times of between one to two days were employed. Although this method results in crystals of improved purity, contamination results from diffusion from the quartz. Further this method has a disadvantage in that the wafers to be employed as substrates have to be sliced from these crystals and during the slicing procedure frequently contamination results.
It is an object of this invention to provide a method for the purification of II-VI semiconductive materials in which substrates of improved purity are produced.
It is a further object of this invention to provide a method for purification of II-VI substrates in which the substrate may be immediately used in a doping operation.
These and other objects of the invention will be apparent from the description that follows.